This application claims the benefit of Korean Application No. 00-11040, filed Mar. 6, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium secondary battery and method of manufacturing thereof, and more particularly, to a lithium secondary battery having improved safety and reliability by preventing explosion of the battery due to thermal runaway.
2. Description of the Related Art
Accompanying the technological development of portable electronic devices which have become miniaturized and lightweight, there has become a high demand for high performance secondary batteries for supplying power to those portable electronic devices.
In accordance with such demand, as a higher energy density battery, lithium secondary batteries have been rapidly developed to substitute for lead batteries or nickel-cadmium batteries.
The lithium secondary batteries have higher energy density and processibility (easily made into a desired shape such as a cylindrical case or a battery pack) and can be easily manufactured, compared to other conventional batteries, thereby being easily adapted for electronic appliances. Therefore, much attention has been paid to the lithium secondary batteries as the most promising type of batteries. A lithium secondary battery generally utilizes lithium nickel oxide, lithium cobalt oxide or lithium manganese oxide as a cathode active material, and carbon, metallic lithium or an alloy thereof as an anode active material thereof. Also, usable electrolytes include a polymer solid electrolyte based on a polymer matrix, such as liquid a organic electrolyte, e.g., polyethylene oxide, polyacrylonitrile or polyvinylidene fluoride.
Lithium secondary batteries are classified according to the kind of electrolyte used, into lithium ion batteries and lithium ion polymer batteries. The lithium ion batteries use a liquid electrolyte, whereas the lithium ion polymer batteries use a gel-type or solid electrolyte.
While the above-described lithium secondary batteries, compared to the other type batteries, have excellent lifetime and energy density characteristics, they may experience local internal shorting in the case where external shock, e.g., nail piercing, is applied thereto. Then, the temperature of the portion where the internal shorting has taken place increases intensively. In particular, if the internal shorting occurs at the active material layer formed on both surfaces of the electrode current collector, the internal temperature of the battery greatly rises. Also, when local shorting occurs, the separator cannot properly exert a shut-down function of suppressing current flow by suppressing migration of ions in the event of an increase in the internal temperature of the battery. Thus, the temperature rise due to local shorting causes thermal runaway, resulting in explosion of a battery.
FIG. 1 is a diagram showing a typical example of a cylindrical lithium ion battery.
Referring to FIG. 1, a cylindrical lithium secondary battery 10 includes a cylindrical case 15 and an electrode assembly 14 installed inside the case 15. Here, the electrode assembly 14 is constructed such that a separator 13 is interposed between a cathode 11 and an anode 12. A cap assembly 16 is connected to the upper portion of the electrode assembly 14.
A process for preparing the aforementioned cylindrical lithium ion battery will now be described.
First, in a state in which the separator 13 is interposed between a cathode plate 11 and the anode plate 12, the resultant is wound around a mandrel in a jelly-roll type configuration to fabricate the electrode assembly 14. Then, the mandrel is removed, the electrode assembly 14 is put into a space of the case 15 and then an electrolytic solution is injected. When injection of the electrolytic solution is completed, the cap assembly 16 is connected to the upper portion of the case 15, thereby completing the lithium ion battery shown in FIG. 1.
As described above, if the mandrel is removed, an empty space is left inside the electrode assembly. Conventionally, a center pin has been inserted into the empty space to assist in maintaining the shape of the electrode assembly.
However, insertion of the center pin has only a trivial effect in maintaining the shape of the electrode assembly 14 or expansion of the electrode assembly 14. Thus, currently, the empty space is allowed to exist at it is, to be used as an electrolytic solution inlet, a tip passage for welding an anode tap, or a gas existence area.
In order to increase the effective space of a battery, a method in which the empty space of a battery is reduced by reducing the diameter of the mandrel, has been proposed. However, according to this method, there is a risk of an electrode plate being cut during winding. Thus, since it is impossible to reduce the diameter of the mandrel to a predetermined level or below, the space remaining after removing the mandrel is necessarily left over.
To solve the above problems, it is an object of the present invention to provide a lithium secondary battery having improved reliability and safety by absorbing internal heat of the battery using an empty space of an electrode assembly to prevent explosion of the battery.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects, there is provided a lithium secondary battery having a winding-type electrode assembly and a case accommodating the electrode assembly, wherein ion-conductive polymer is contained in at least one of a hollow portion of the electrode assembly and an inner space of the case other than the hollow portion.
The ion-conductive polymer is preferably a material gelled by a non-aqueous electrolytic solution.